Hey guys. How are you reinforcing the backs of your molds? I figure flatter molds you can cut some strips of wood and screw then together and mix up some resin putty and bond on the back, but how about more complex shapes? Or is my first idea flawed?
Pictures would help too if you would share. Trying to figure this out since I may need to make a mold where there’s no flexing.
I generally just use cardboard tubes cut in half and then cut into sections to go around curvature. Stick them down with hot melt glue before laminating over them with glass.
You want to be very careful doing this if you are doing elevated temperatures at all though, as you can get severe print through of the backing structure onto the mould surface. Depends on what your mould looks like, but I often just put the backing structure on the flanges rather than anywhere over the part surface itself. I prefer making moulds monolithic if possible just to avoid the potential for any print through.
Cool. Yeah I am planning to just cut up some mdf board and screw it together and then mix up some putty. Never thought of the tubes but that’s a good idea too.
Trouble with using wood is that it is quite moisture sensitive, so it will swell with moisture which can cause problems. So just make sure you completely seal it.
Foam insulation for pipes. They are temperature and moisture uncritically and easy to drape. If you laminate over them they are just kind of a lost core.
Won’t they dissolve in vinyl water resin or no?
No, they are made from PVC. They are fine with all standard resins.
For bigger moulds that need to be very stiff, you can make cored panels and cut them to shape. Just do not put and filled resins between them and the backside of the mould surface. Otherwise it will create hard points that will likely print threw. Its best to purposely leave a gap to avoid this
I think I get what you are saying but want to be clear from your pics.
You’re saying pretty much build a small box that all runs around the flange area on the backside and then reinforce the middle with a rib section that’s floating to not print threw to the mold surface?
The second picture kind of confuses me because it looks like it’s laid with carbon and infused to completely seal the supports.
Yes, exactly, you should make sure that the structure is not in direct contact with back side of laminate. Second photo you see is final product, after +/-45 tape used to adhere the structure to laminate, hand laid without vacuum. By the way, this structure is a bit of an overkill for this mould size, but very effective and light. Bottom line is you want to increase cross section thickness. Making a box, or ribs as others have describe help do this quickly without having to make a super thick laminate.
I’ve had very good results using epoxy granite on the back of my molds. Just mix epoxy with one of the sand and gravel bags from Home Depot. Epoxy granite is incredibly hard, strong and stiff. Plus it makes epoxy go a lot further.
Sounds heavy. I may have to start working out again if that’s as heavy as it sounds!
Epoxy granite is heavy but not really that different from metal molds.
The last epoxy granite mold I made is approx 1.5" x 6" x 25" and I can lift it easily. It would be a problem for large molds though. I made my CNC machine gantry with epoxy granite and carbon fiber. It’s 8"x 48" x 24" and I had to buy a shop crane to lift it onto the table.
Maybe i’ll Give it a try.
How thick do you reinforce the back of your molds? And do you have a ratio and recommended epoxy to use?
I think you will find making a simple structure much easier, faster, effective and lighter. If its a small mould, doing some king of mineral casting can work, but then again heating and cooling that mass will be a pig. Not to mention CTE of the tool. So lots of things to consider really.
It really depends on the size of the mold. Longer molds require more reinforcement than short wide ones. For my 6" x 24" and 6" x 30" molds, I have been using around 1/2" to 1" of epoxy granite over the back of a carbon fiber shell. It’s an extremely effective combo that is quick and easy to make and incredibly strong and stiff. It’s also very cost effective.
I started using epoxy granite / carbon fiber molds because I got sick of breaking my other molds when parts got stuck. It is the best combo I have found to date.
The ratio of epoxy granite mix to resin will depend on how heavily filled your tooling resin is to start with. Rather than trying to follow a rule, I prefer to just mix my resin and then slowly add the granite / sand mix until I achieve the desired viscosity. I like it to be a dry clay-like consistency which can be added like putty. I paint a thin layer of resin between the carbon fiber shell and the epoxy granite mix to make sure it bonds properly.
This is just not correct. If you are having those issues, it is because you are doing something wrong. I have used a lot of epoxy granite for parts large and small without any such issues.
I’m not sure what you consider to be a simple structure but I am sure that most people will be capable of mixing resin and sand and applying it to the back of a mold like putty. I can’t think of any other effective method that is less complex than that.
If you are having heating / cooling issues, I suggest trying it with a slower cure resin or, if it is for an oven cure process, use a heat cure resin that sets (slowly) at room temp before post curing.
I never said it didn’t work for you, nor couldn’t work for someone else’s applications. Point being is that there are many things to consider when making tooling. If you think these things don’t matter, and should not be consider, you’re wrong. Everything I mentioned are key points to tooling design. Concluding that it works for you does not mean it will work for everyone else’s applications. And typically you design the tooling around the part, not the part around the tooling.
I can’t see how that relates to what we were talking about - I.e.the performance and ease of use with epoxy granite.
The texture, consistency and the ability to vary them based on the use makes it easy to use for any type or shape of mold. I.e. It can be easily spread over the most complex curves and applied like paste to vertical surfaces.
I agree that there are various factors to consider when designing a mold. It’s just that most of you said about epoxy granite was wrong. It’s not about whether it worked for me or it being a matter of opinion. The info was / is incorrect.
As a suggestion, if you know a better method, you should share it. None of us are too old to learn a better way. Just saying building a simple structure is going to be easier and more effective doesn’t mean anything. A simple structure made of what? And how is it better or easier than epoxy granite?
I already described a fairly simple, common means to build a backing structure to a tool on the first page. If you want the only criteria to be speed, then I could agree mineral casting is faster. I can also agree there are many applications that it will work on just fine, as previously stated. However adding a thick layer of mineral casting will decrease the tools K-value, meaning it will take longer for the tool to heat up and cool down. Second, small moulds are ok for your casting, but get anything large and you cant get the same stiffness you would from a traditional backing structure, strength to weight ratio. Granite epoxy casting will have low strength, and you will not be able to build cross section thickness up enough to create stiffness without adding a lot of weight. Further you would need a cradle to hold your tool in a desired position unless the tool sits well naturally. With a backing structure you consider this in design. Lastly, you’re creating an unbalanced tool, with different CTE materials, which in large structure will likely see warping or deviation from nominal target at high temps. If this works for you and others that’s great, but I only see this being beneficial for small, room/low temp cure tools, that you want to whip out without doing much lamination or cutting.